Current Mode Controller

The present disclosure relates to a current mode controller for a switching converter.

Current mode control is a commonly used control method for switched-mode power supplies (SMPS).

Compared to the voltage control method, current mode control has a high frequency bandwidth, which results in improved control loop dynamics and leads to a better line noise rejection, by reducing the small-signal dynamics from second-order to first-order. This simplifies the design of the feedback circuit.

The current control method can be classified into average current mode and peak current mode. As the name suggests, the average current mode control adjusts the average current of the inductor. Peak current mode is a control technique where the duty cycle is terminated when the inductor current reaches a threshold level determined by an external voltage controller.

It is desirable to provide an improved current mode controller for a switching converter, when compared with known systems.

According to a first aspect of the disclosure there is provided a current mode controller for a switching converter comprising one or more power switches, an energy storage element, and being configured to receive an input voltage and to generate an output voltage, the current mode controller comprising an error voltage generator configured to generate an error voltage that is dependent on the output voltage and a reference voltage, a current sensing module configured to sense a current that is flowing through the energy storage element, receive the error voltage, and generate a slope compensated sensed current signal that is dependent on the sensed current and the error voltage, an error amplifier configured to receive the error voltage, generate an amplified voltage error signal by applying a first amplification coefficient to the error voltage, the first amplification coefficient being dependent on the error voltage, and a control signal generator configured to generate one or more control signals to control the switching operation of the one or more power switches of the switching converter, each of the one or more control signals being dependent on the slope compensated sensed current signal and the amplified voltage error signal.

Optionally, the energy storage element comprises an inductor.

Optionally, the current mode controller is configured to provide peak current control.

Optionally, the error voltage generator comprises a subtractor circuit configured to generate the error voltage by subtracting the reference voltage from the output voltage or by subtracting the output voltage from the reference voltage.

Optionally, the current sensing module comprises a current sensing amplifier configured to sense the current that is flowing through the energy storage element, and generate an amplified sensed current signal by applying a second amplification coefficient to the sensed current, wherein the slope compensated sensed current signal is dependent on the amplified sensed current signal, thereby being dependent on the sensed current.

Optionally, the current sensing module comprises a slope generation module configured to receive the error voltage, and generate a slope compensation signal that is dependent on the error voltage, wherein the slope compensated sensed current signal is dependent on the slope compensation signal, thereby being dependent on the error voltage.

Optionally, the energy storage element comprises an inductor, and the slope compensation signal is dependent on i) a differential inductor voltage, ii) the second amplification coefficient, iii) the inductance of the inductor, iv) a current rise time, and v) a dynamic duty cycle that is dependent on the reference voltage, the input voltage and the error voltage.

Optionally, the current sensing module comprises an addition circuit configured to generate the slope compensated sensed current signal by adding the slope compensation signal and the amplified sensed current signal, and provide the slope compensated sensed current signal to the control signal generator.

Optionally, the slope compensation signal is added to the amplified sensed current signal by adding a bias current to the amplified sensed current signal.

Optionally, the error amplifier comprises dynamic gain module configured to receive the error voltage, and generate the first amplification coefficient using the error voltage.

Optionally, the energy storage element comprises an inductor, and the first amplification coefficient is dependent on i) a differential inductor voltage, and ii) a dynamic duty cycle that is dependent on the reference voltage, the input voltage and the error voltage.

Optionally, the error amplifier comprises a voltage error amplifier module configured to receive the error voltage, receive the first amplification coefficient from the dynamic gain module, and generate the amplified voltage error signal by applying the first amplification coefficient to the error voltage.

Optionally, the control signal generator comprises a comparator configured to receive the slope compensated sensed current signal at a first input terminal, receive the amplified voltage error signal at a second input terminal, generate a comparator output signal at an output terminal, the comparator output signal being dependent on the comparison between the sensed current signal and the amplified voltage error signal, and a pulse width modulator circuit configured to receive the comparator output signal, and generate the one or more control signals using the comparator output signal.

Optionally, a duty cycle is approximately equal to the reference voltage divided by the input voltage, for the duty cycle being greater than or approximately equal to 50% the current sensing module is configured to sense the current that is flowing through the energy storage element, receive the error voltage, and generate the slope compensated sensed current signal that is dependent on the sensed current and the error voltage, the control signal generator is configured to generate the one or more control signals to control the switching operation of the one or more power switches of the switching converter, each of the one or more control signals being dependent on the slope compensated sensed current signal and the amplified voltage error signal, and for the duty cycle being less than 50% the current sensing module is configured to sense the current that is flowing through the energy storage element, generate a sensed current signal that is dependent on the sensed current, and the control signal generator is configured to generate the one or more control signals to control the switching operation of the one or more power switches of the switching converter, each of the one or more control signals being dependent on the sensed current signal.

Optionally, the switching converter is a buck converter, a boost converter or a buck-boost converter.

According to a second aspect of the disclosure there is provided a switched-mode power supply comprising a switching converter configured to receive an input voltage and to generate an output voltage, the switching converter comprising one or more power switches, an energy storage element, and a current mode controller comprising an error voltage generator configured to generate an error voltage that is dependent on the output voltage and a reference voltage, a current sensing module configured to sense a current that is flowing through the energy storage element, receive the error voltage, and generate a slope compensated sensed current signal that is dependent on the sensed current and the error voltage, an error amplifier configured to receive the error voltage, generate an amplified voltage error signal by applying a first amplification coefficient to the error voltage, the first amplification coefficient being dependent on the error voltage, and a control signal generator configured to generate one or more control signals to control the switching operation of the one or more power switches of the switching converter, each of the one or more control signals being dependent on the slope compensated sensed current signal and the amplified voltage error signal.

Optionally, the energy storage element comprises an inductor.

Optionally, the current mode controller is configured to provide peak current control.

Optionally, the switching converter is a buck converter, a boost converter or a buck-boost converter.

It will be appreciated that the switched-mode power supply of the second aspect may include features set out in relation to the first aspect and/or may include other features as described herein.

According to a third aspect of the disclosure there is provided a method of controlling a switching converter comprising one or more power switches, an energy storage element, and being configured to receive an input voltage and to generate an output voltage, the method comprising generating an error voltage that is dependent on the output voltage and a reference voltage, using an error voltage generator, sensing a current that is flowing through the energy storage element using a current sensing module, receiving the error voltage at the current sensing module, generating a slope compensated sensed current signal that is dependent on the sensed current and the error voltage using the current sensing module, receiving the error voltage at an error amplifier, generating an amplified voltage error signal by applying a first amplification coefficient to the error voltage using the error amplifier, the first amplification coefficient being dependent on the error voltage, and generating one or more control signals using a control signal generator, each of the one or more control signals being dependent on the slope compensated sensed current signal and the amplified voltage error signal, and controlling the switching operation of the one or more power switches using the one or more control signals.

It will be appreciated that the method of the third aspect may include providing and/or using features set out in the first and/or second aspect and can incorporate other features as described herein.

Peak current mode control has advantages over other control methods, such as simple cycle-to-cycle current limiting and good current sharing in a multiphase system. That said, there are several disadvantages of peak current control. For example, the control may become unstable if the duty cycle exceeds 50%, leading to subharmonic oscillations sensitivity to noise, especially for small inductor current ripples.

1 FIG. 100 102 shows a graphof the inductor current as it varies with time for a practical implementation of a switching converter operating with a duty cycle greater than 50%, and a graphof the inductor current as it varies with time for a practical implementation of the switching converter operating with a duty cycle less than 50% (Raghavan Sampath, Digital Peak Current Mode Control of Buck Converter Using MC56F8257 DSC, Document Number: AN4716, Freescale Semiconductor, Inc., 2013).

1 FIG. 100 The examples presented inrelate to a peak current mode controlled switching converter using a continuous conduction mode (CCM). The inductor current may transition to sub-harmonic oscillations depending on the operating conditions. For example, perturbations in the inductor current (for example due to fluctuations in a control voltage) persist when the duty cycle exceeds 50%, as shown in the graphwhere the inductor current moves away from the steady state inductor current profile with each switching cycle. Perturbations in the inductor current diminish with each switching cycle when the duty cycle is less than 50%.

A common approach to restoring stability is to apply slope compensation in peak current controlled converters with duty cycles above 50%. One drawback is that the saw peak current control method produces an error in the output voltage relative to the reference voltage.

2 FIG. 200 is a timing graphshowing simulation results for a practical implementation of a buck converter with slope compensation and a practical implementation of a buck converter without slope compensation. In both examples the duty cycle is 33%.

202 204 206 208 The following traces are shown on the timing graph: an output voltage of the buck converter with slope compensation (a trace), an output voltage of the buck converter without slope compensation (a trace), an inductor current of the buck converter with slope compensation (a trace), and an inductor current of the buck converter without slope compensation (a trace).

For a larger output current, there is a larger error. The magnitude of this error can be reduced by increasing the loop gain of the switching converter, but this will lead to a decrease in system stability. Another disadvantage is that when stabilizing the system using the slope compensation method, the error of the output voltage relative to the reference voltage increases proportionally to the slope compensation value.

3 FIG. 300 302 304 306 302 308 310 312 314 316 318 320 304 322 324 326 328 is a schematic of a known switch mode power supplycomprising a current mode controllerand a switching convertercoupled to a load. The current mode controllercomprises a comparator, a PWM modulator, a current sense amplifier, and addition circuit, a slope generator, a voltage error amplifierand an addition circuit. The switching convertercomprises a power traincomprising a high side switchand a low side switch, and an output filter.

300 322 322 316 The switch mode power supplyuses slope compensation. The control loop includes two main feedback signals: the first feedback signal is a voltage error signal, which represents the difference between the sensed output voltage and reference voltage; and the second feedback signal is the sensed output current of the power train. For duty cycles less than 50%, the first and second feedback signals are input directly to the control block, which determines the duty cycle needed to maintain output voltage regulation. However, for duty cycles greater than 50%, slope compensation is implemented. A bias current is added to the sensed output current of the power train. The amount of bias current is determined by the current slope generator. The slope of the bias current remains constant over the entire operating range. Although this method is effective in avoiding sub-harmonic oscillations maintaining loop stability, the disadvantages include increased output voltage error and poor dynamic load response.

A generally known calculation formula for slope compensation, considering a differential inductor voltage Vl is as follows:

where Ksense is a transformation coefficient for inductor current to voltage conversion, L is the inductance of the inductor of the switching converter, D is the duty cycle and ton is the current rise time.

V(ton) is the slope compensation voltage as a function of ton. For dead-beat control, the slope compensation voltage V(ton) is set to the slope of the de-magnetizing phase of the inductor current.

The current rise time ton is as follows:

where Tsw is the switching period.

The duty cycle D for a switching converter using continuous conduction mode (CCM) may be calculated as follows:

where Vref is a reference voltage and Vin is the input voltage.

4 FIG. 400 402 402 322 is a schematic of a switch mode power supplycomprising a dynamic slope moduleto provide dynamic slope compensation. The dynamic slope moduleadjusts the bias current added to the sensed output current of the power trainbased on the voltage error signal.

5 FIG. 500 502 500 502 is a schematic of a current mode controllerand a switching converter, in accordance with a first embodiment of the present disclosure. The current mode controllerand the switching convertermay form a switched-mode power supply.

502 502 504 502 506 502 The switching convertercomprises one or more power switches. In the present embodiment, the switching convertercomprises a power switch. The switching converterfurther comprises an energy storage element, which may, for example, comprise an inductor. The switching convertermay, for example be a buck converter, a boost converter or a buck-boost converter.

504 504 Each of the one or more power switchesmay comprise a transistor, such as a metal oxide semiconductor field effect transistor (MOSFET). For each power switch, the MOSFET may be p-type or n-type in accordance with the understanding of the skilled person.

502 504 500 500 During operation, the switching converterreceives an input voltage Vin and generates an output voltage Vout, with the output voltage Vout being generated as a result of the switching of the one or more power switches, as controlled by the current mode controller. The current mode controllermay provide peak current control.

500 508 The current mode controllercomprises an error voltage generatorconfigured to generate an error voltage Verror that is dependent on the output voltage Vout and a reference voltage Vref.

500 510 1 510 1 506 The current mode controllerfurther comprises a current sensing modulethat is configured to receive the error voltage Verror. During operation, the current sensing modulesenses a current Iflowing through the energy storage element.

510 2 1 1 The current sensing modulethen generates a slope compensated sensed current signal Isigthat is dependent on the sensed current Iand the error voltage Verror.

500 512 1 2 1 1 The current mode controllerfurther comprises an error amplifierthat is configured to receive the error voltage Verrorand to generate an amplified voltage error signal Verrorby applying a first amplification coefficient to the error voltage Verror. The first amplification coefficient is dependent on the error voltage Verror.

500 514 502 2 2 The current mode controllerfurther comprises a control signal generatorthat configured to generate one or more control signals to control the switching operation of the one or more power switches of the switching converter. Each of the one or more control signals is dependent on the slope compensated sensed current signal Isigand the amplified voltage error signal Verror.

514 516 504 In the present example, the control signal generatorgenerates a control signalto control the switching operation of the switch.

506 Each of the one or more control signals may be a digital signal that switches between a high state and a low state. A digital control signal being provided to a switch may result in the switching of the switch. For example, when the digital signal in a high state is provided to the switch, the switch may be turned to an “on state” where current flow is permitted, and when the digital signal in a low state is provided to the switch, the switch may be turned to an “off state” where current flow is prevented. Through such a switching operation, the energy storage elementmay be repeatedly coupled to and decoupled from a power source (for example as provided by an input voltage Vin), which results in the generation of the output voltage Vout.

Each of the control signals may be provided via a gate driver configured to set the voltage as provided to the switch to a suitable level for controlling the switching operation of the switch.

502 It will be appreciated that other switching convertercontrol schemes may be performed, in accordance with the understanding of the skilled person.

500 During operation, the current mode controllermay provide both dynamic slope compensation and dynamic gain control.

510 2 1 1 1 510 Dynamic slope compensation may be provided by the current sensing modulewhich provides the slope compensated current signal Isigthat is dependent on the error voltage Verrorin addition to the sensed current I. Based on the value of the output voltage error Verror, the current sensing moduledynamically adjusts the slope compensation curve. This improves voltage regulation and dynamic load response.

512 512 1 Dynamic gain control may be provided by the error amplifierwhere the gain of the error amplifierprovided by the first amplification coefficient has a dependency on the error voltage Verror.

1 512 512 Based on the value of the output voltage error Verror, the error amplifieradjusts the gain of the voltage error amplifier. This further improves voltage regulation and dynamic load response.

500 502 500 500 510 1 1 1 514 516 For a specific embodiment of the current mode controllerand the switching converter, the duty cycle may be determined using equation (3). Dynamic slope compensation may be provided by the current mode controllerwhen operating with a duty cycle that is greater than 50%. Dynamic slope compensation may not be provided by the current mode controllerwhen operating with a duty cycle that is less than 50%. For example, when the duty cycle is less than 50%, the current sensing modulemay sense the current Iand generated a sensed current signal that is dependent on the sensed current I, and has no dependency on the error voltage Verror. The control signal generatorthen generates the one or more control signalsthat are dependent on the uncompensated sensed current signal.

6 FIG. 500 502 is a schematic of a specific embodiment of the current mode controllerand a specific embodiment of the switching converter, in accordance with a second embodiment of the present disclosure.

508 600 1 The error voltage generatormay comprise a subtractor circuitthat is configured to generate the error voltage Verrorby subtracting the reference voltage Vref from the output voltage Vout, or by subtracting the output voltage Vout from the reference voltage Vref.

510 602 602 1 506 1 1 2 1 1 The current sensing modulemay comprise a current sensing amplifier. During operation, the current sensing amplifiersenses the current Ithat is flowing through the energy storage elementand generates an amplified sensed current signal Isigby applying a second amplification coefficient to the sensed current I. The slope compensated sensed current signal Isigis dependent on the amplified sensed current signal Isigand is therefore dependent on the sensed current I.

510 604 1 1 2 1 The current sensing modulemay further comprise a slope generation moduleconfigured to receive the error voltage Verrorand generate a slope compensation signal Vslope that is dependent on the error voltage Verror. The slope compensated sensed current signal Isigis dependent on the slope compensation signal Vslope, thereby being dependent on the error voltage Verror.

604 608 610 608 610 316 402 The slope generation modulemay comprise a slope generatorand a dynamic slope moduleto provide dynamic slope compensation. The slope generatorand the dynamic slope modulemay function substantially as described for the slope generatorand the dynamic slope module, respectively, in accordance with the understanding of the skilled person.

510 612 612 2 1 612 2 514 1 1 The current sensing modulemay further comprise an addition circuit. During operation, the addition circuitmay generate the slope compensated sensed current signal Isigby adding the slope compensation signal Vslope and the amplified sensed current signal Isigtogether. The addition circuitmay then provide the slope compensated sensed current signal Isigto the control signal generator. The slope compensation signal Vslope may be added to the amplified sensed current signal Isigby adding a bias current to the amplified sensed current signal Isig.

512 614 1 1 The error amplifiermay comprise a dynamic gain modulethat is configured to receive the error voltage Verrorand generate the first amplification coefficient using the error voltage Verror.

512 616 1 2 1 The error amplifiermay further comprise a voltage error amplifier modulethat is configured to receive the error voltage Verror, receive the first amplification coefficient, and generate the amplified voltage error signal Verrorby applying the first amplification coefficient to the error voltage Verror.

514 618 2 2 618 620 620 2 2 The control signal generatormay comprise a comparatorthat is configured to receive the slope compensated sensed current signal Isigat a first input terminal and receive the amplified voltage error signal Verrorat a second input terminal. The comparatoris further configured to generate a comparator output signalat an output terminal, with the comparator output signalbeing dependent on the comparison between the compensated sensed current signal Isigand the amplified voltage error signal Verror.

514 622 620 516 620 The control signal generatormay further comprise a pulse width modulation circuitconfigured to receive the comparator output signaland to generate the one or more control signalsusing the comparator output signal.

502 624 504 626 504 626 502 506 630 506 506 The switching convertermay comprise a power traincomprising the switchand a switch. The switchmay be a high side switch and the switchmay be a low side switch. The switching convertercomprises the energy storage elementcoupled to a load. The energy storage elementmay comprise an inductor. The energy storage elementmay be referred to as an “output filter”.

1 The slope compensation signal Vslope may be dependent on a differential inductor voltage Vl, the second amplification coefficient Ksense, the inductance L of the inductor, a current rise time ton, and a dynamic duty cycle DD that is dependent on the reference voltage Vref, the input voltage Vin and the error voltage Verror. The relationship between the parameters may be as follows, by rewriting equation (1):

604 The slope generation modulemay generate the slope compensation signal Vslope, in accordance with equation (4).

The dynamic duty cycle DD may be written as follows:

2 The amplified voltage error signal Verrormay be written as follows:

where Kvdyn is the first amplification coefficient.

The main expression describing the current control method with slope compensation is as follows:

1 6 FIG. where Il is the current Ias illustrated in, and Ksense is the second amplification coefficient.

1 For constant first amplification coefficient Kvdyn, second amplification coefficient Ksense, and current Il, the addition of slope compensation Vslope will result in a increase in the error voltage Verror.

1 To compensate for the increasing error voltage Verror, embodiments of the present disclosure enable the first amplification coefficient Kvdyn to be variable. For example, the first amplification coefficient may be dependent on the differential inductor voltage Vl and the dynamic duty cycle DD, for example, as follows:

where Kv is a base expression of the amplification coefficient, and Kn is a normalising coefficient.

618 Equation (8) describes the state of the signals at the inputs of the comparatorat the switching instant, and may be referred to as the “command law” for the controller.

The differential voltage on the inductor Vl, may be calculated if the input and output voltage are known. The differential inductor voltage Vl may be calculated with an error that is compensated for. The system may be implemented as a behavioural model or in the digital domain in the form of synthesized code, for the determination of the differential inductor voltage Vl.

In summary, the present embodiment relates to a digital slope compensation apparatus for a switched-mode power supply that uses dynamic slope compensation and a dynamic gain coefficient. A sensor used for sensing an inductor current of the switched-mode power supply, a comparator is used for generating a trigger signal according to a comparison of the error between the output voltage and the reference voltage and the inductor current, and a pulse width modulator is used for controlling the operation of a switched-mode power supply, wherein the pulse width modulator is arranged to be triggered by the trigger signal of the comparator. The use of dynamic slope compensation and a dynamic gain coefficient provides improvements over known systems.

7 FIG. 3 FIG. 4 FIG. 6 FIG. is a timing graph showing simulation results for practical implementations of pulsed power supplies with static slope compensation (for example as shown in), for dynamic slope compensation (for example as shown in), and for dynamic slope compensation and dynamic amplification coefficient (for example as shown in). The reference voltage Vref is 2V, the over current protection is 8 A, and the load is 5 A.

700 702 704 706 708 710 There is shown an inductor current for the system using static slope compensation (a trace), an inductor current for the system using dynamic amplification coefficient (a trace), an inductor current for the system using dynamic slope compensation and dynamic amplification coefficient (a trace), an output voltage for the system using static slope compensation (a trace), an output voltage for the system using dynamic amplification coefficient (a trace), an output voltage for the system using dynamic slope compensation and dynamic amplification coefficient (a trace).

8 FIG. 3 FIG. 4 FIG. 6 FIG. is a further timing graph showing simulation results for practical implementations of pulsed power supplies with static slope compensation (for example as shown in), for dynamic slope compensation (for example as shown in), and for dynamic slope compensation and dynamic amplification coefficient (for example as shown in).

800 802 804 806 808 810 812 814 816 There is shown: the slope compensation signal Vslope for the system using static slope compensation (a trace), the slope compensation signal Vslope for the system using dynamic amplification coefficient (a trace), the slope compensation signal Vslope for the system using dynamic slope compensation and dynamic amplification coefficient (a trace), the dynamic duty cycle DD for the system using static slope compensation (a trace), the dynamic duty cycle DD for the system using dynamic amplification coefficient (a trace), the dynamic duty cycle DD for the system using dynamic slope compensation and dynamic amplification coefficient (a trace), the first amplification coefficient Kvdyn for the system using static slope compensation (a trace), the first amplification coefficient Kvdyn for the system using dynamic amplification coefficient (a trace), the first amplification coefficient Kvdyn for the system using dynamic slope compensation and dynamic amplification coefficient (a trace).

9 FIG. 614 1 2 618 is a schematic of a specific embodiment of the dynamic gain module. In a specific embodiment, the first amplification coefficient KVdyn may be calculated by digital methods in a digital unit of the switch-mode power supply. In the digital unit the error value Verroris multiplied by the dynamic gain coefficient (KVdyn), as represented by equation (6), and then the finished result Verroris output to a digital to analog converter (DAC), then the output of the DAC goes to the comparator.

3 FIG. For embodiments of the present disclosure dynamic slope compensation provides improved load response and over-all output regulation over a wide load range as compared to known slope compensation methods such as presented in. Known slope compensation techniques employ a current compensation bias with a fixed slope over the entire operating range (duty cycle>50%). Embodiments of the present disclosure may use dynamic slope compensation, which may use an adaptive current compensation bias with a variable slope.

Embodiments of the present disclosure provide improvement through reduction of such parameters as: time response, transient voltage drop and error of the output voltage relative to the reference voltage.

Embodiments of the present disclosure use dynamic slope compensation together with a dynamic amplification coefficient. Dynamic slope compensation may be provided by adding an additional component to the calculation of the slope compensation function, that is proportional to the error of output voltage relative to the reference voltage. The dynamic amplification coefficient may be provided by generating an error in the output voltage relative to the reference voltage to make variable by the same law as provided for dynamic slope compensation. The combination of dynamic slope compensation and dynamic gain coefficient may be used to accelerate load response and reduce error of the output voltage in systems which work with duty cycles more than 50%.

Dynamic slope compensation adaptively adjusts the slope compensation signal, the resulting duty cycle of the converter will differ from prior art solutions. To determine if a converter is practicing the invention, connect the converter to a variable load. Increase the load in incremental steps and record the duty cycle of the power stage. Pay particular attention to the load settings that result in duty cycles greater than 50%. Based on these duty cycle measurements, it can be determined if the slop compensation signal plus dynamic gain control is a fixed or variable slope. If it is determined to be variable beyond what would normally be associated with slope compensation only, the converter is practicing dynamic slope compensation.

Common reference numerals and variable between figures represent common features. Various improvements and modifications may be made to the above without departing from the scope of the disclosure.